ICAO says itīs ok to say the callsign at anytime during the transmission.
A few years ago our flt-safety dept. looked at RT procedures, and came up with the "callsign-first" procedure/recommendation/idea (and, no, they donīt kill you, if you donīt do it this way).
Itīs supposed to have several advantages (you decide!):
1. itīs easy: You just read back what ATC said.
2. by saying the callsign first, ATC immediatly knows itīs the correct aircraft answering.
3. during the readback of an altitude clearance, it gives the PF enough time to dial the new alt. The PNF is then supposed to simply read it from the FMA (and thus actually check what the aircraft/autopilot is supposed to do - redundancy), which is mostly at the end of the sentence.
It all sounds much more complicated or maybe even dogmatic than it is, and I actually just changed my RT a few months ago.(the young FOs have been trained to do it this way from the start,I guess)
It took a bit of concentration at first, but I must say, itīs just as easy as doing it any other way and you get the "advantages" on top.
Iīve even started punching in new freqs in the MCDU-scratchpad before readback and then dialing them - makes it much easier for me at least - no rt-button pushing and talking while dialing and looking at the radio.
Hope it explains what some of our people are doing...